In the operation of a coal-fired electricity plant, there is produced an exhaust gas flow which is contaminated with compounds containing sulphur, in particular oxidized sulphur compounds. In other processes, for example in the manufacture of sinters needed for charging in the blast furnace process in the iron and steel industry, waste gas flows containing also sulphur occur. These waste gas flows represent a significant environmental hazard, in that their release into the atmosphere causes acidification of the environment. Consequently, production installations in which such gas flows occur are usually provided with apparatus for removing the compounds containing sulphur from the gas flow. However, such gas cleaning systems are relatively costly and often just shift the environmental problem from one place to another. One known example of this is the system in which compounds containing sulphur are removed from a waste gas bound to chalk thereby creating a large problem of disposal.
EP-A-217567 is mentioned as describing treatment of a gas containing a large proportion of SO.sub.2 to prepare it for the Claus reaction which generates elementary sulphur.
NL-A-7505940 describes concentration of sulphur dioxide from a waste gas containing sulphur dioxide by absorbing the sulphur dioxide and then regenerating a gas flow containing sulphur dioxide at higher concentration. The latter gas flow, before being fed to a Claus reactor, is mixed with a reducing gas which is generated from the waste gas from the Claus reactor.